The present invention relates a light barrier system with a radiation source and a radiation sensor, acted on by the radiation of said source and having at least two sensor elements, and including means for polarizing the radiation differently in two branches, and different polarization radiation filters in front of the two sensor elements for detecting the intensity of radiation in the two respective branches, and wherein the two sensor elements are connected in a gating circuit which emits a signal on different irradiation intensities being received by the two sensor elements.
Light barriers are known, e.g., from DE No. 1,934,321 or DE No. 2,014,107 and serve preferably to protect against intrusion. As soon as radiation, (which is preferably in the infrared or visible spectral region), emitted by a radiation source and directed towards a radiation sensor, is interrupted, e.g., by the body of an intruder or by being covered during an attempt at sabotage, a gating circuit releases an alarm signal.
The polarization of the radiation emitted by the radiation source and the arrangement of an equivalent polarizing filter in front of one of the sensor elements, (such that the other sensor element receives radiation which is not or is differently polarized), results in the gating circuit not emitting an output signal when some other radiation, e.g., solar radiation or diffused light, impinges upon the radiation sensor, the polarization of which is different from that of the light barrier radiation. Alternatively the said other radiation may not be polarized, in which event both sensor elements are acted on equally.
Light barriers of this kind, if properly designed, may also be utilized for outside application during daylight hours. For such use the discrimination against interference can be improved by employing alternating radiation of a defined frequency and by tuning the frequency of the gating circuit to synchronize with that of the radiation source as is known in the art. To provide synchronization, the radiation source is typically connected with the radiation sensor or the gating circuit, with the radiation source and radiation sensors located proximate to each other and a reflector is located at a distance from the source and sensors to reflect radiation from the source back to the sensors. However, a reflector is very sensitive to soiling and misalignment, so that the range of such a light barrier, i.e., the safely controllable length of the distance monitored, is therefore rather limited.
Light barriers for outside applications with a larger range, extending from about 10 meters to more than 100 meters, however, are influenced by the weather since fog and rain drops cause radiation scatter, so that when weather conditions are unfavorable, a noticeable weakening of the radiation received will occur, particularly when the path of the radiation between radiation source and radiation sensor is large. In order to prevent the release of a faulty alarm signal in such a case, the sensitivity of the gating circuit must be reduced correspondingly. Moreover in case of a weather-conditioned enlargement of the monitoring beam resulting from radiation scatter, the weakening of the radiation becomes smaller as a result of an object, e.g., an intruder, so that at unfavorable weather conditions, an intruder can no longer be recognized since sufficient scattered radiation still impinges upon the sensor.